Under the Securities Exchange Act of 1934
(Amendment No. 9)*

(Name of Issuer)

(Title of Class of Securities)

(CUSIP Number)

(Name, Address and Telephone Number of Person
Authorized to Receive Notices and Communications)

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of Sections 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box o.

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

* The remainder of this cover page shall be filled out for a reporting person's initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed to be "filed" for the purposes of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

(1) The beneficial ownership percentage is based upon 150,720,962 issued and outstanding shares as reported by the Issuer in the prospectus supplement it filed with the Securities and Exchange Commission on January 28, 2014.

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(2) The beneficial ownership percentage is based upon 150,720,962 issued and outstanding shares as reported by the Issuer in the prospectus supplement it filed with the Securities and Exchange Commission on January 28, 2014.

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Introductory Statement

This Amendment No. 9 (this “Amendment”) amends the Schedule 13D initially filed with the Securities and Exchange Commission on August 6, 2004, as previously amended on September 7, 2005, November 2, 2005, May 20, 2009, June 18, 2010, April 20, 2011, December 15, 2011, August 10, 2012 and February 14, 2014 (the “Original Schedule 13D”) relating to shares of the Common Stock, $0.001 par value per share (the “Common Stock”), of Idenix Pharmaceuticals, Inc., a Delaware corporation (the “Issuer”).  All information reported in the Original Schedule 13D remains in effect except to the extent that it is amended or superseded by information contained in this Amendment.  Capitalized terms used but not defined in this Amendment shall have the respective meanings set forth with respect thereto in the Original Schedule 13D.

No change except as described below.

(a) - (c) and (f). The name, business address, present principal occupation or employment and citizenship of the executive officers and members of the Board of Directors of each of the Reporting Persons is set forth on Schedule I hereto and is incorporated herein by reference.

(d) and (e). Neither the Reporting Persons nor, to the best knowledge of each of them, any of the persons listed on Schedule I hereto with respect to each such Reporting Person during the last five years, (i) has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors) or (ii) was a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

No change except as described below.

Board of Directors

On March 28, 2014, Mr. Anthony Rosenberg, a designee of Novartis Pharma AG (“Novartis Pharma”) to the Issuer’s board of directors under the Restated Stockholders Agreement, submitted his resignation from the Issuer’s board of directors, effective immediately prior to the Issuer’s 2014 Annual Meeting of Stockholders (including any adjournment thereof) scheduled for June 5, 2014. As of the date hereof, Novartis Pharma does not intend to designate a replacement candidate for Mr. Rosenberg but it retains the right to do so under the Restated Stockholders Agreement.

The disclosure in Item 5(a)-(b) of the Original Schedule 13D is hereby amended as follows.

Novartis AG (“Novartis”) and Novartis Pharma disclaim beneficial ownership of the 71,500 shares that Novartis Pharma had the right to acquire under the Restated Stockholders Agreement. This disclaimer is based on the waiver by Novartis Pharma of such right in respect of such shares.

Novartis Pharma is the record and beneficial owner of 33,320,674 shares of Common Stock shares of Common Stock, representing 22.1% of the outstanding shares of Common Stock.  Novartis Pharma has shared power to vote or direct the vote and shared power to dispose or to direct the disposition of all shares of Common Stock owned of record by it.

Novartis is the beneficial owner of all shares of Common Stock beneficially owned by Novartis Pharma and has shared power to vote or direct the vote and shared power to dispose of or to direct the disposition of all shares of Common Stock as to which Novartis Pharma has such right or rights.

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SIGNATURE

After reasonable inquiry and to the best knowledge and belief of the undersigned, the undersigned hereby certify that the information set forth in this statement is true, complete and correct.

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SCHEDULE I — for Form 13-D/A

DIRECTORS AND EXECUTIVE OFFICERS OF
NOVARTIS AG AND NOVARTIS PHARMA AG

DIRECTORS AND OFFICERS OF NOVARTIS AG

The name, address, citizenship and present principal occupation or employment of each of the directors and executive officers of Novartis AG are set forth below. Unless otherwise indicated below, (i) each occupation set forth opposite an individual’s name refers to employment with Novartis AG and (ii) the business address of each director and executive officer of Novartis AG is Novartis Campus, Lichtstrasse 35, CH-4056, Basel, Switzerland.

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DIRECTORS AND OFFICERS OF NOVARTIS PHARMA AG

The name, address, citizenship and present principal occupation or employment of each of the directors and executive officers of Novartis Pharma AG are set forth below. Unless otherwise indicated below, (i) each occupation set forth opposite an individual’s name refers to employment with Novartis Pharma AG and (ii) the business address of each director and executive officer of Novartis Pharma AG is Novartis Campus, Lichtstrasse 35, CH-4056, Basel, Switzerland.

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JOINT FILING AGREEMENT

In accordance with Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended, each of the persons named below agrees to the joint filing of a Statement on Schedule 13D (including amendments thereto) with respect to the common stock, par value $0.001 per share, of Idenix Pharmaceuticals, Inc., a Delaware corporation.

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